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Full text of "A brief account of radio-activity"

A BRIEF ACCOUNT OF 
RADIO-ACTIVITY 



BY 

FRANCIS P. VENABLE, PH.D., D.Sc., LL.D. 

PROFESSOR OF CHEMISTRY, UNIVERSITY OF NORTH CAROLINA 

AUTHOR OF 
"A SHORT HISTORY OF CHEMISTRY," 

"PERIODIC LAW," ETC. 



D. C. HEATH & CO., PUBLISHERS 

BOSTON NEW YORK CHICAGO 



V4- 



COPYRIGHT, 1917, 
BY D. C. HEATH & Co. 

IA 7 



PREFACE 

I HAVE gathered the material for this little book because 
I have found it a necessary filling out of the course for my 
class in general chemistry. Such a course dealing with the 
composition and structure of matter is left unfinished and 
in the air, as it were, unless the marvellous facts and deduc- 
tions from the study of radio-activity are presented and 
discussed. The usual page or two given in the present 
text-books are too condensed in their treatment to afford 
any intelligent grasp of the subject, so I have put in book 
form the lectures which I have hitherto felt forced to give. 

Perhaps the book may prove useful also to busy men in 
other branches of science who wish to know something of 
radio-activity and have scant leisure in which to read the 
larger treatises. 

It is needless to say that there is nothing original in the 
book unless it be in part the grouping of facts and order of 
their treatment. I have made free use of the writings of 
Rutherford, Soddy, and J. J. Thomson, and would here 
express my debt to them just a part of that indebtedness 
which we all feel to these masters. I wish also to acknowl- 
edge my obligations to Professor Bertram B. Boltwood for 
his helpful suggestions in connection with this work. 



m 



37169G 



CONTENTS 

CHAPTER I 

DISCOVERY OF RADIO-ACTIVITY PAGE 

The beginning Radio-active bodies An atomic property , 
Discovery of new radio-active bodies Discovery of Polonium 
Discovery of Radium Other radio-active bodies found . . 1 

CHAPTER II 

PROPERTIES OF THE RADIATIONS 

lonization of Gases Experimental confirmation Application of 
electric field Size and nature of ions Photographing the 
track of the ray Action of radiations on photographic plates 
Discharge of electrified bodies Scintillations on phosphorescent 
bodies Penetrating power Magnetic deflection Three 
types of rays Alpha rays Beta rays Gamma rays 
Measurement of radiations Identifications of the rays ... 7 

CHAPTER III 
CHANGES IN RADIO-ACTIVE BODIES 

Radio-activity a permanent property Induced activity Discovery 
of Uranium X Conclusions drawn Search for new radio- 
active bodies Methods of investigation Nature of the 
radiations Life-periods Equilibrium series 17 

CHAPTER IV 
NATURE OF THE ALPHA PARTICLE 

Disintegrating of the elements Identification of the rays The 
alpha rays Alpha rays consist of solid particles Electrical 
charge Helium formed from alpha particles Discovery of 
Helium Characteristics of Helium Table of constants ... 25 



vi CONTENTS 

CHAPTER V 

THE STRUCTURE OF THE ATOM 

Properties of Radium Energy evolved by radium Necessity for a 
disintegration theory Disintegration theory Constitution 
of the atom Rutherford's atom Scattering of alpha particles 

Stopping power of substances 32 

CHAPTER VI 

RADIO-ACTIVITY AND CHEMICAL THEORY 

Influence upon chemical theory The periodic system Basis of the 
periodic system Influence of positive nucleus Determination 
of the atomic number Use of X-ray spectra Changes caused 
by ray-emission Atomic weight losses Lead the end product 

Changes of position in the periodic system Changes from 
loss of beta particles Isotopes Radio-activity in nature 
Radio-active products in the earth's crust Presence in air and 

soil waters Cosmical radio-activity 41 

INDEX . .... 53 



A BRIEF ACCOUNT OF 
RADIO-ACTIVITY 



A BEIEF ACCOUNT OF 
EADIO-ACTIVITY 

CHAPTER I 
DISCOVERY OF RADIO-ACTIVITY 

THE object of this brief treatise is to give a simple 
account of the development of our knowledge of radio- 
activity and its bearing on chemical and physical 
science. Mathematical processes will be omitted, as 
it is sufficient to give the assured results from calcu- 
lations which are likely to be beyond the training of 
the reader. Experimental evidence will be given in 
detail wherever it is fundamental and necessary to a 
confident grasp of some of the marvelous deductions 
in this new branch of science. Theories cannot be 
avoided, but the facts remain while theories grow old 
and are discarded for others more in accord with the 
facts. 

As so often happens in the history of science, the 
opening up of this new field with its fascinating dis- 
The closures was due to an investigation under- 

Beginning taken for another purpose but painstakingly 
carried out with a mind open to the truth wherever it 
might lead. 



2 RADIOACTIVITY 

In 1895, Rontgen modestly announced his discovery 
of the X rays. This attracted immediate and intense 
interest. Among those who undertook to follow up 
these phenomena was Becquerel, who, because of the 
apparent connection with phosphorescence, tried the 
action of a number of phosphorescent substances upon 
the photographic plate, the most striking characteristic 
of the X rays being their effect upon such sensitive 
plates. In these experiments he obtained no results 
until he tried salts of uranium, recalling previous 
observations of his as to their phosphorescence. Dis- 
tinct action was noted. Furthermore, he proved that 
this had no connection with the phenomenon of phos- 
phorescence, as both uranic and uranous salts were 
active and the latter show no phosphorescence. Bec- 
querel announced his discoveries in 1896 and this was 
the beginning of the new science of radio-activity. 

The rays given off by uranium and its salts were 
found to differ from the X rays. They showed no 
Radio-active appreciable variation in intensity, no pre- 
Bodies vious exposure of the substance to light 

was necessary, and neither changes of temperature 
nor any other physical or chemical agency affected 
them. 

At first uranium and its compounds were the only 
known source of these new radiations, but many other 
substances were examined and two years later thorium 
and its compounds were added to the list. In general 
the discharging action seemed about the same. Other 



DISCOVERY 3 

elements and ordinary substances show a minute 
activity. Only potassium and rubidium have a greater 
activity than this, and theirs is only about one- 
thousandth that of uranium. 

In the examination of uranium and thorium com- 
pounds it was found that the activity was determined 
An Atomic by the uranium and thorium present; it 
Property was proportional to the amount of these 
elements present and independent of the nature of the 
other elements composing the compound. The con- 
clusion was, therefore, that the activity was an inherent 
property of the atoms of uranium and thorium, that is, 
an atomic property. This was a long step forward and 
introduced into science the conception of a new prop- 
erty of matter, or at least of certain forms of matter. 
In examining a large number of minerals contain- 
ing uranium and thorium, Mme. Curie made the 
Discovery of important observation that many of these 
New Radio- were more active than the elements them- 
ies selves. In measuring the activity she 
made use of the electrical method which will be de- 
scribed later. In the following table giving her results 
for uranium minerals the numbers under i give the 
maximum current in amperes. They serve simply for 
comparison. 

t 

Pitchblende from Joachimsthal 7.0 X 10 ~ u 

Clevite 1.4 X 1Q- 11 

Chalcolite 5.2 X 1Q- 11 

Autunite . . .2.7x10 ~ u 



4 RADIO-ACTIVITY 



Carnotite 6.2 x 1Q- 11 

Uranium 2.3 x 10 ~ n 

Uranium and potassium sulphate . . . 0.7 X 10 ~ u 
Uranium and copper phosphate 0.9 X 10 ~ u 

The last three are pure uranium and compounds of 
that element given for comparison with the first five, 
which are naturally occurring minerals. The last com- 
pound has the same composition as chalcolite and is 
simply the artificially prepared mineral. It has the 
activity which would be calculated from the propor- 
tion of uranium present, the copper and phosphoric 
acid contributing no activity. 

Since the activity is not dependent upon the composi- 
tion but upon the amount of uranium present, the activ- 
ity in all of the minerals should be less than that of 
uranium. On the contrary, it is several times greater. 
Natural and artificial chalcolite also show a marked 
difference in favor of the former. The supposition 
was a natural one, therefore, that these minerals con- 
tained small quantities of an element, or elements, 
undetected by ordinary analysis and having a much 
greater activity than uranium. Similar results were 
obtained in the examination of thorium minerals and 
thorium salts. 

Following up this supposition, M. and Mme. Curie 
set themselves the task of separating this unknown sub- 
Discove-y of stance. Starting with pitchblende, a sys- 
Poionium tematic chemical examination was made. 
This is an exceedingly complex mineral, containing 



DISCOVERY 5 

many elements. The processes were laborious and 
demanded much time and minute care. They need 
not be described here. It is sufficient to say that along 
with bismuth a very active substance was separated, to 
which Mme. Curie gave the name of polonium for 
Poland, her native land. Its complete isolation is 
very difficult and sufficient quantities of the pure sub- 
stance have not been obtained to determine its atomic 
weight and other properties, but some of the lines of 
its spectrum have been determined. Chemically it 
is very closely analogous to bismuth. 

In a similar manner a barium precipitate was ob- 
tained from pitchblende which contained a highly 
Discovery of active substance. The pure chloride of this 
Radium body and barium can be prepared together 

and then separated by fractional crystallization. To 
the new body thus found the name of radium was 
given. It is similar in chemical properties to barium. 
Its atomic weight has been determined by several care- 
ful investigators and is accepted as 226. Its spectrum 
has been mapped and its general properties are known. 
It is a silvery white, oxidizable metal. In one ton of 
pitchblende about 0.2 gram of radium is present; this 
is about 5000 times greater than the amount of polonium 
present. The activity of the products was depended 
upon as the guide in these separations. The radium 
found is relatively enormously more active than the 
pitchblende or uranium. 

In the above separations use was made of relation- 



6 RADIO-ACTIVITY 

ships to bismuth and barium. Similarly, by taking 
other Radio- advantage of chemical relationship to the 
active Bodies iron group of elements, another body was 
partially separated by Debierne, to which 
he gave the name actinium. Bolt wood discovered in 
uranium minerals the presence of a body which he 
named ionium, and which is so similar to thorium 
that it cannot be separated from it. It, however, far 
exceeds thorium in activity. 

The lead which is present in uranium and thorium 
minerals apparently in fairly definite ratio to the 
amount of uranium and thorium is found, on separa- 
tion and purification, to possess radio-active properties. 
This activity is due to the presence of a very small 
proportion of an active constituent called radio-lead, 
which has chemical properties identical with those of 
ordinary lead. The bulk of the lead obtained from 
radio-active minerals differs in atomic weight from 
ordinary lead and appears also to be different according 
to whether its source is a thorium or a uranium mineral. 

A large number of other radio-active substances have 
been separated and some of their properties determined, 
but these were found by different means and will be 
noted in their proper place. They number in all more 
than thirty. The sources or parents of these are the 
original uranium or thorium, and the products form 
regular series with distinctive properties for each 
member. 



CHAPTER II 

PROPERTIES OF THE RADIATIONS 

THE activity of these radio-active bodies consists in 
the emission of certain radiations which may be sepa- 
rated into rays and studied through the phenomena 
which they cause. 

One of these phenomena is the power of forming ions 
or carriers of electricity by the passage of the rays 
lonization through a gas, thus ionizing the gas. The 
of Gases details of an experiment will serve to make 
the meaning of this ionization clear. 



t 



v 

+V 
FIG. 1. IONIZATION OF GASES. 

When this apparatus is set up a minute current will 
be observed without the introduction of any radio- 
active matter. This, as Rutherford says, has been 
found due mainly to a slight natural radio-activity of 
the matter composing the plates. If radio-active mat- 



8 RADIO-ACTIVITY 

ter is spread on plate A, which is connected with one 
pole of a grounded battery, and if plate B is connected 
with an electrometer which is also connected with the 
earth, a current is caused which increases rapidly with 
the difference of potential between the plates, then 
more slowly until a value is reached that changes only 
slightly with a larger increase in the voltage. 

According to the theory of ionization, the radiation 
produces ions at a constant rate. The ions carrying a 
positive charge are attracted to plate B, while those 
negatively charged are attracted to plate A, thus caus- 
ing a current. These ions will recombine and neutralize 
their charges if the opportunity is given. The num- 
ber, therefore, increases to a point at which the ions 
produced balance the number recombining. 

When an electric field is produced between the plates, 
the velocity of the ions between the plates is increased 
in proportion to the strength of the electric field. In a 
weak field the ions travel so slowly that most of them 
recombine on the way and consequently the observed 
current is very small. On increasing the voltage the 
speed of the ions is increased, fewer recombine, the 
current increases, and, when the condition for recom- 
bination is practically removed, it will have a maximum 
value. This maximum current is called the saturation 
current and the value of the potential difference required 
to give this maximum current is called the saturation 
P.D. or saturation voltage. 

The picture, then, is this. The radiations separate 



PROPERTIES OF THE RADIATIONS 9 

the components of the gas into ions, or carriers of elec- 
tricity, half of which are charged negatively and half 
positively. In the electric field those negatively 
charged seek the positive plate and those positively 
charged seek the negative plate. If time is given, these 
ions meet and recombine, their charges are neutralized, 
and there is no current. 

This theory of the ionization of gases has been most 
interestingly confirmed by direct experiment. For 
Experimental instance, the ions may form nuclei for the 
Confirmation condensation of water, and in this way the 
existence of the separate ions in the gas may be shown 
and the number present actually counted. 

When air saturated with water vapor is allowed to 
expand suddenly, the water present forms a mist of 
small globules. There are always small dust particles 
in air and around these as nuclei the drops are formed. 
These drops will settle and thus by repeated small 
expansions all dust nuclei may be removed and no 
mist or cloud will be formed by further expansions. 

If now the radiation from a radio-active body be 
introduced into the condensation vessel, a new cloud 
is produced in which the water drops are finer and more 
numerous according to the intensity of the rays. On 
passing a strong beam of light through the condensa- 
tion chamber, the drops can readily be seen. These 
drops form on the ions produced by the radiation. 

If the condensation chamber has two parallel plates 
for the application of an electric field like that already 



10 RADIO-ACTIVITY 

described, the ions will be carried at once to the elec- 
trodes and disappear. The rapidity of this action 
depends upon the strength of the electric field and 
Application of experiment shows that the stronger the field 
Electric Field foe smaller the number of condensation 
drops formed. If there is no electric field, a cloud can 
be produced some time after the shutting off of the 
source of radiation, showing that time is required for the 
recombination of the ions. 

If the drops are counted (there being special methods 
for this) and the total current carried accurately 
Size and Na- measured, then the charge carried by each 
ture of ions ; on may fa ca l cu l a t e d. This has been 
determined. The mass of an ion compared with the 
mass of the molecules of gas in which it was produced 
can also be approximately estimated. In the study 
of these ions the view has been held that the charged 
ion attracted to itself a cluster of molecules which sur- 
rounded the charged nucleus and traveled with it. It 
is roughly estimated that about thirty molecules of 
the gas cluster around each charged ion. 

Utilizing the fact that these ions with their clusters 
of molecules form nuclei for the condensation of water 
Photo a h vapor, C. T. R. Wilson has by instantaneous 
ing the Track photography been able to photograph the 
track of an ionizing ray through air. The 
number of the ions produced, and hence the number of 
drops, is so great that the trail is shown as a continuous 
line. u. In the copy of this photograph it will be seen 



PROPERTIES OF THE RADIATIONS 11 

that at some distance from its source the straight trail 
is slightly but abruptly bent. Near the end of its 
course there is another abrupt and 
much sharper bend. These bends 
show where the ionizing ray, in this 
case an alpha particle, has been de- 
flected by more or less direct collis- 
ion with an atom. These collisions 
and the final disappearance of the 
ray will be discussed later. 

Taking up now other means of ex- 
amining these radiations, it is well to 
consider their action upon 

Action of Ra- . , . . 

diations on a photographic or sensi- 
tive plate. It will be re- 
called that this was the FIG. 2. PHOTOGRAPH 
method by which their existence was F J^* OF AN 
originally detected. To illustrate the 
method, the following account of how one such photo- 
graph was taken may be given. 

The plate was wrapped in two thicknesses of black 
paper. The objects were placed upon this and the 
radio-active ore, separated by a board one inch thick, 
was placed above. The exposure lasted five days. The 
action is much less rapid and the result not so clearly 
defined as in the case of photographs taken by X rays. 
Of course, the removal of the board and the use of 
more concentrated preparations of radium would give 
quicker and better results. The method, however, on 




12 RADIO-ACTIVITY 

account of time consumed and lack of definition is ill 
adapted to accurate work. 

The radiations from radio-active bodies can discharge 
both positively and negatively electrified bodies by 
Discharge of ma ^ m g tne ^ surrounding them a con- 
Electrified ductor of electricity. To demonstrate this, 
use is made of an electroscope. If the 
hinged leaf of such an instrument be electrically charged 
and a radio-active body be brought into its neighbor- 
hood, the electricity will be discharged and the leaf 
return to its original position. The rapidity of this 
discharge is used to measure the degree of activity of 
the body giving off the radiation. 




FIG. 4. GOLD-LEAF ELECTROSCOPE. 

The gold-leaf L is attached to a flat rod R and is insulated inside the 
vessel by a piece of amber S supported from the rod P. The system is 
charged by a bent rod CC' passing through an ebonite stopper. After 
charging, it is removed from contact with the gold-leaf system. The rods 
P and C and the cylinder are then connected with the earth. 






I 



\ 



FIG. 3 PHOTOGRAPH OF VARIOUS OBJECTS 

TAKEN BY MEANS OF PITCHBLENDE 



PROPERTIES OF THE RADIATIONS 13 

It was found by Crookes that a screen covered with 

phosphorescent zinc sulphide was brightly lighted up 

. . when exposed to the radiations. This is 

Scintillations . . 

on Phosphor- due to the bombardment ot the zinc sulphide 

escent Bodies ^ & type Q my ^^ ^ a j pha ray Under 

a magnifying glass this light is seen to be made up of 
a number of scintillating points of light and is not con- 
tinuous, each scintillation being of very short duration. 
By proper subdivision of the field under the lens, the 
number of scintillations can be counted with close 
accuracy. 

A simple form of apparatus called the spinthariscope 
has been devised to show these scintillations. A zinc 
sulphide screen is fixed in one end of a small tube and 
a plate carrying a trace of radium is placed very close 
to it. The scintillations can be observed through an 
adjustable lens at the other end of the tube. Outer 
light should be cut off, as in a dark room. The 
screen then appears to be covered with brilliant flashes 
of light. Other phosphorescent substances, such as 
barium platino-cyanide, may be substituted for the 
zinc sulphide, but they do not answer so well. 

By penetrating power is meant the power exhibited 
by the rays of passing through solids of different thick- 
Penetrating nesses and gases of various depths. This 
Power power varies with different radiations and 

with the nature of the solid or gas. For instance, a 
sheet of metallic foil may be used and the effect of 
aluminum will differ from that of gold and the different 



14 RADIO-ACTIVITY 

rays vary in penetrating power. In the case of gases air 
will differ from hydrogen, and it is noticed that certain 
rays disappear after penetrating a short distance, while 
others can penetrate further before being lost. 

If the radiations are subjected to the action of a 
strong magnetic field, it is found that part of them 
Magnetic are much deflected in the magnetic field 
Deflection an( j d escr i De circular orbits, part are only 
slightly deflected and in the opposite direction from 
the first, and the remaining rays are entirely unaffected. 




FIG. 5. SHOWING MAGNETIC DEFLEC- 
TION OF a, /3, AND 7 RAYS. 

By the use of these methods of investigation it is 
learned that the radiations consist of three types of 
Three Types rays. These have been named the alpha, 
of Rays beta, and gamma rays, respectively. Some 
radio-active bodies emit all three types, some two, 
and some only one. The distinguishing characteristic 
of these types of rays may be summed up as follows : 



PROPERTIES OF THE RADIATIONS 15 

The alpha rays have a positive electrical charge and 

a comparatively low penetrating power. They are 

slightly deflected in strong magnetic and 

electric fields. They have a great ionizing 

power and a velocity about one-fifteenth that of light. 

. The beta rays are negatively charged and have a 

greater penetrating power than the alpha rays. They 

show a strong deflection in magnetic and 

Beta Rays . . - is i j 

electric fields, have less ionizing power than 
the alpha rays, and a velocity of the same order as 
light. 

The gamma rays are very penetrating and are not 

deflected in the magnetic or electric fields. 

Gamma Rays . . . 

Ihey have the least ionizing power and a 
very great velocity. 

The penetrating, power of each type is complex and 
varies with the source, so the statements given are 
but generalizations. The alpha rays are projected 
particles which lose energy in penetrating matter. As 
to the power of ionizing gases, if that for the a rays 
is taken as 10,000, then the /3 rays would be approxi- 
mately 100 and the 7 rays 1. 

The rays are examined and measured in several 
ways: 1. By their action on the sensitive photo- 
Measurement S ra P mc plates. The use of this method is 
of Radia- laborious, consumes time, and for com- 
parative measurements of intensity is un- 
certain as to effect. 

2. By electrical methods, using electroscopes, quad- 



16 RADIO-ACTIVITY 

rant electrometers, etc. These are the methods most 
used. 

3. By exposure to magnetic and electric fields, 
noting extent and direction of deflection. 

4. By their relative absorption by solids and gases. 

5. By the scintillations on a zinc sulphide screen. 
The alpha rays have been identified as similar to the 

so-called canal rays. These were first observed in the 
identification study of the X rays. When an electrical 
of the Rays discharge is passed through a vacuum tube 
with a cathode having holes in it, luminous streams 
pass through the holes toward the side away from the 
anode and the general direction of the stream. They 
travel in straight lines and render certain substances 
phosphorescent. These rays are slightly deflected by 
a magnetic field and in an opposite direction from that 
taken by the cathode rays in their deflection. The 
rays seem to be positive ions with masses never less 
than that of the hydrogen atom. Their source is 
uncertain, but they may be derived from the electrodes. 

The beta rays are identical in type with the cathode 
rays and are negative electrons. 

The gamma rays are analogous to the X rays and 
are of the order of light. They are in general consider- 
ably more penetrating than X rays. For example, the 
gamma rays sent out by 30 milligrams of radium can 
be detected by an electroscope after passing through 
30 centimeters of iron, a much greater thickness than 
can be penetrated by the ordinary X rays. 



CHAPTER III 

CHANGES IN RADIO-ACTIVE BODIES 

Is this power of emitting radiations a permanent 

property or is it lost with the passage of time? The 

first investigations of the activity of uranium 

acfa>fty 10 a and thorium showed no loss of intensity at 



Permanent fa e en( j o f seV eral years, and radium also 
seemed to show no decrease in its enormous 
activity. Polonium, however, was found to lose most 
of its activity in a year, and later it appeared that some 
radio-active substances lost most of their activity in 
the course of a few minutes or hours. 

A phenomenon called induced or secondary radio- 
activity was also observed. Thus a metal plate or 
induced AC- wire exposed to the action of thorium oxide 
tivity or gome hours became itself active. This 

induced activity was not permanent but decreased to 
half its value in about eleven hours and practically 
disappeared within a week. Similar phenomena were 
observed when radium was substituted for thorium. 

In 1900 Crookes precipitated a solution of an active 
uranium salt with ammonium carbonate. The pre- 
Discovery of cipitate was dissolved so far as possible in 
Uranium x an excess o f the reagent, leaving an insoluble 
residue. This residue was many hundred times more 



18 RADIO-ACTIVITY 

active, weight for weight, than the original salt, and 
the solution containing the salt was practically inactive. 
At the end of a year the uranium salt had regained its 
activity while the residue had become inactive. 

Another method of obtaining the same result is to 
dissolve crystallized uranium nitrate in ether. Two 
layers of solution are formed, one ether and the other 
water coming from the water of crystallization. The 
aqueous layer is active, while the water layer is inactive. 
Similarly, by adding barium chloride solution to a solu- 
tion of a salt of uranium and then precipitating the 
barium as sulphate, the activity is transferred to this 
precipitate. These experiments give proof of the 
formation and separation of a radio-active body by 
ordinary chemical operations. 

So, too, in the case of thorium salts a substance can 
be obtained by means of ammonium hydroxide which 
is several thousand times more active than an equal 
weight of the original salt. After standing a month, 
the separated material has lost its activity and the 
thorium salt has regained it. Here, again, there is the 
formation, separation, and loss of a radio-active body. 

Now, these are ordinary chemical processes for the 
separation of distinct chemical individuals. The re- 
Conciusions suits, therefore, lead naturally to the con- 
Drawn elusions: (1) it would seem that uranium 
and thorium are themselves inactive and the activity 
is due to some other substance formed by these ele- 
ments; (2) this active substance is produced by some 



CHANGES IN RADIO-ACTIVE BODIES 19 

transformation in those elements, for on standing the 
activity is regained. This latter conclusion is startling, 
for it indicates a change in the atom which, up to the 
time of this discovery, was deemed unchangeable under 
the influence of such physical and chemical changes as 
were known to us. 

The search for new radio-active bodies and the study 

of their characteristics has been systematically and 

successfully carried on. The bodies ob- 

Search for . J 

New Radio- tamed in the above experiments were 
active Bodies named uran i um x and thorium X, respec- 
tively. Further, it became clear from the investiga- 
tion of uranium minerals that radium, polonium, 
actinium, and ionium originated from uranium. From 
thorium minerals a body was separated called meso- 
thorium, which was analogous to radium. Both tho- 
rium and radium were found to give off a radio-active 
gas. The first lost half of its activity in less than one 
minute. The second was more stable and lost half of 
its activity in about four days. The name radium 
emanation was given to the latter and it was found 
chemically and physically to belong to the class of mon- 
atomic or noble gases, such as helium, argon, neon, etc., 
which had been discovered by Ramsay. In some cases 
the chemical action was determined and these new 
bodies were found analogous to well-known elements, as 
radium to barium, polonium to bismuth. The physical 
properties were investigated and, where possible, spectra 
were mapped and atomic weights determined. 



20 RADIO-ACTIVITY 

It is clear, therefore, that these bodies are elemental 
in character and as such are made up of distinct, similar 
atoms, just as the commonly recognized elements are 
believed to be. In this way more than thirty new 
elements have been added to the list. These new ele- 
ments are called radio-active elements, but it is an 
open question whether all atoms do not possess this 
property in greater or less degree. Certainly, it is 
possessed in varying degree by four of the old elements 
widely separated in the Periodic System, namely, 
uranium, thorium, rubidium, and potassium. The 
last two, while feebly active themselves, do not form 
any secondary radio-active substance so far as is known. 
Only two of the elements, then, can definitely be said 
to go through these transformations. It is just possible 
that radio-activity may be found to be a common 
property of all atoms and of all matter. 

It is important to know how these new bodies were 
discovered and distinguished from one another. Two 
Methods of properties are relied upon. One is the 
investigation nature of the rays emitted and the other is 
the duration of the activity. Of course, knowledge of 
the physical and chemical properties is also of great 
importance whenever obtainable. 

The nature of the radiation is a distinguishing charac- 
w ^ , teristic, though similarity here does not 

nature 01 

the Radia- prove identity of substances. Some emit 

a rays only, some emit /3 rays, some emit 

two of the possible rays, as for instance, /3 and 7, and 



CHANGES IN RADIO ACTIVE-BODIES 21 

some emit all three. The rays may also differ in 
the velocity with which they are emitted by different 
radio-active substances. Thus, in the case of one 
substance the a rays may have a slightly greater or 
less penetrating power than those emitted by some 
other substance, and this may be true also of the 
other rays. 

The duration of the activity is called the life period. 

This is absolutely fixed for each body and furnishes 

the most important mode of differentiating 

Life Periods e 

among them. It measures the relative 
stability and is the time which must elapse before their 
activity is lost and they, changing into something 
else, entirely disappear. The measure usually adopted 
is the half -value period. Two hypotheses are made 
use of: 

1. That there is a constant production of fresh radio- 
active matter by the radio-active body. 

2. That the activity of the matter so formed de- 
creases according to an exponential law with the time 
from the moment of its formation. 

These hypotheses agree with the experimental results. 
The decrease and rise of activity, for example, of 
uranium and uranium X, and also of thorium and tho- 
rium X, have been measured, plotted, and the equations 
worked out. 

Manifestly, a state of equilibrium will be reached 
when the rate of loss of activity of the matter already 
produced is balanced by the activity of the new matter 



22 RADIO-ACTIVITY 

produced. This equilibrium and the knowledge of the 
rate of decrease in general will have little value if this 
rate, like chemical changes, is subject to the influence 
of chemical and physical conditions. The rate of 
decrease has been found to be unaltered by any known 
chemical or physical agency. For instance, neither 
the highest temperatures applicable nor the cold of 
liquid air have any appreciable effect. 

In order to measure the disintegration of a radio- 
active body in units of time so that the rate may be 
Equilibrium comparable with that of other radio-active 
Series bodies, the relation between the amounts 

under consideration must be a definite one. For 
this purpose equal weights of the bodies are not 
taken, but use is made of the amounts which are 
in equilibrium with a fixed amount of the parent 
substance. 

One gram of radium has been settled upon as the 
standard for that series and a unit known as the "curie" 
has been adopted to express the equilibrium quantity 
of radium emanation. Thus, a curie of radium emana- 
tion (or niton) is the weight (or, as this is a gas, the 
volume at standard pressure and temperature) of the 
emanation in equilibrium with one gram of radium. 
This, by calculation and experiment, is found to be 0.63 
cubic millimeter. When this amount has been pro- 
duced by one gram of radium, the formation and decay 
will exactly balance one another. This is, therefore, 
one curie of emanation. 



CHANGES IN RADIO-ACTIVE BODIES 23 

The measurement of the rate of decay is difficult but 
can be carried out with great accuracy, even down to 
seconds, in the case of certain short-lived bodies. 
Errors crept in at first from the failure to completely 
separate the substances produced in the series, and 
sometimes because of the simultaneous production of 
two substances. 

As stated, the decay follows an exponential law. 
The time required for the decay of activity to half- 
value does not mean, therefore, that there will be total 
decay in twice that time. Thus the half -value period 
for uranium X is about 22 days. The period for com- 
plete decay is about 160 days. This half -value period 
corresponds to the half-value recovery period of ura- 
nium, which is also 22 days. 

These were the earlier figures obtained for uranium 
X and they illustrate some of the difficulties surrounding 
such determinations. It was found later that the body 
examined as uranium X was really a constant mixture 
and of course the decay and recovery periods were also 
composite. It required later and very skilful work to 
separate them into the bodies indicated in the disinte- 
gration series. 

The half -value period for thorium X is much shorter, 
namely, a little over four days, and this is also the 
recovery period for thorium X. The plotted decay 
and recovery curves will intersect at this point. 

The consecutive disintegration series, with the half- 
value periods, for the uranium and thorium series as 



24 RADIO-ACTIVITY 

given by Soddy are seen in the following tables. They 
are probably subject to some changes on further and 
more accurate determination. The nature of the rays 
emitted is also given. 



^v * 




Thorium /*~v , x 


Uranium (*J -n 




(4xl0 10 pw-'t)->O ^l 7 ' 


(8 x 10 9 years) \-S*O<* 




years ?) ^jp 


Uranium X (L^J\ -> /3y 
(35.5 days) \*_y 
I fft) 


^ P \ 


Mesothoriumi /^^N 
(7.9 years) \*J 


1 

Ionium J!L 
(5x10* to ((QUo^ 
10* years) \-S 


Actinium (?) \) 

Radio- x^k 
Actinium ( J-^O 01 
(28.1 days) J^.^j 


Mesothorium2 /^ . . v u 
(8.9 hours) V^y ^ " 

4 

Radiothorium /^.^Q, 
(2.91 years ?) \_J u 


Radium fC^Uo*. 


Actinium X ^^-*O <x 


Thorium X ^"^-^n ^ 


(2,500 years) V_y 


(15 days) x_x 


(5.35 days) vy 


J 


4 


1 


Emanation |M\-O ot 
(5.57 days) Vl/ 


Emanation ^"\ _ 
(5.6 seconds) V-/^ 


Emanation JJ_>Q oc 
(70 seconds) v f 


Radium A /(Quo c 


Actinium A ^~\-n x 


Thorium A i*\*O <?<! 


(4.3 minutes) \1/ 


(0.0029 second) W^^ 


(0.203 second) V^ w 


4 


1 


4, 


Radium B (T^)W ft}) 


Actinium B [VtfJ 


Thorium B (j ->o ^p) 


(38.5 minutes) v_x 


(52. 1 minutes) V^ 
I 


(15.3 hours) V 


Radium Ci f/7^-O ^ 
(28.1 minutes) I \_X^ 


Actinium Ci f (~\+Q oc 
(3.10mins.) 1 V-/ 


Thorium Ci f f^Uo 
(79 minutes) 1 V_y 


Radium C 2 1 /O\_v ., 
(1.9 minutes) tv*/: p / 


Actinium Cz 1 /^^N^Q ^ 


Thorium Cz 1 ^\^Q ^ 


jT 


1 


1 


Radium D ^j^-*/i9) 
(24 years ?) VHx 


Actinium D (j^^^ygexy 
(7.4 minutes) x_x 


Thorium D f~ > )-*/3*y 

(4.5 minutes) ^x * 


Radium E /C7^i_k*A.w 
(7.25 days) (^ ->/*>/ 


Actinium E /^~N 
(unknown) V^y 


I 

Thorium E (^\ 
(unknown) v_x 


Radium F Jk 






(Polonium 0'<> i))-* O <>< 






202 days) V' 






Radium G /f~\ 






(probably V"'*) 






lead) ^^ 







FIG. 6. DISINTEGRATION SERIES FOR URANIUM, ACTINIUM, 
AND THORIUM, AS GIVEN BY SODDY. 



CHAPTER IV 
NATURE OF THE ALPHA PARTICLE 

A THE remarkable disintegrations related in the last 

chapter, in which the heaviest known elementary atom 

that of uranium (at. wt. 238) is by 

Dismtegra- . v . ' J 

tion of the successive stages changed into others ot 
lower atomic weight, afford a clue to the 
nature of the atom and to that goal of the chemist, the 
final constitution of matter. The composite nature 
of the atom and some sort of interrelation of the 
elements had previously been made apparent from 
a study of the Periodic System and data gathered 
still earlier, but all attempts at working out a so- 
called genesis of the elements had proved vague and 
unsatisfactory. 

To get an understanding of the disintegration occur- 
ring in radio-active substances, the nature of the rays 
identification produced must be known. These' rays are 
of the Rays tne cause o f the activity and their emission 
accompanies the changes or disintegration. They have 
for the sake of convenience been called the alpha, beta, 
and gamma rays. The gamma rays have been identi- 
fied with the X rays discovered by Rontgen and are a 
form of energy analogous to light. The beta rays are 
particles of negative electricity or electrons. With 



26 RADIO-ACTIVITY 

these, then, we have some degree of familiarity. But 
what are the alpha rays? An answer to this question 
should make clearer the character of the changes taking 
place, and should give some insight into the composition 
and mechanism of the atom. 

It has already been stated that these alpha rays are 
similar or analogous to the canal rays, but this advances 
The Alpha the matter very little, as the nature of these 
Rays canal rays has not been fully determined. 

The full identity with them, if proved, should have an 
important theoretical bearing. 

In the first place, these alpha rays have been found 

to be made up of solid particles, that is, of what we 

are accustomed to call matter. Since it 

Consist <>r has become more and more difficult to draw 

Solid Parti- a c j ear distinction between matter and 

cles 

energy, it would perhaps be better to say 
that these particles appear to have some of the prop- 
erties hitherto attributed solely to matter. The best 
evidence that these particles are of atomic mass is 
furnished by their deflection in electric and magnetic 
fields. 

It is not of first importance to discuss this or other 
proofs of the material nature of these particles. That 
Electrical they carry a charge of positive electricity is, 
charge however, a fact of very great import. The 

value of this charge has been carefully determined by 
a number of investigators working with different 
sources of the alpha particles and has been found to 



NATURE OF THE ALPHA PARTICLE 27 

be 9.3 X 10~ 10 electrostatic units (.000,000,000,93 e.s.). 
From the consideration of the charge upon an electron 
previously obtained by J. J. Thomson and others, it 
was concluded that the alpha particle carried two 
unit positive charges; the fundamental unit charge, 
therefore, is half this value, or 4.65 X 10~ l e.s. 

To determine the nature of the alpha par- 
Formed from ticle a crucial experiment was carried out by 
tides* Par " Rutherford and Royds, which was described 

as follows : 

A large quantity of radium emanation was com- 
pressed into a fine glass tube A, 
about 1.5 cm. long. This tube, 
which was sealed to a larger capil- 
lary tube B, was sufficiently thin 
to allow the alpha particles from 
the emanation and its products to 
pass through, but sufficiently thick 
to withstand atmospheric pressure. 
The thickness of the glass wall 
was in most cases less than .01 mm. 
On introducing the emanation into 
the tube, the escape of the alpha 
particles from the emanation was 
clearly seen by the scintillations FlG - 7. APPARATUS 

USED IN EXPERIMENT 

produced at some distance on a BY RUTHERFORD AND 
zinc sulphide screen. After this ROYDS. 
test the glass tube A was surrounded by a glass tube T 
and a small spectrum tube V attached to it. The tube 




28 RADIO-ACTIVITY 

T was exhausted to a charcoal vacuum. By means of 
the mercury column H, the gases in the tube T could 
at any time be compressed into the spectrum tube V 
and the nature of the gases which had been produced 
determined spectroscopically. It was found that two 
days after the introduction of the emanation into A 
the spectrum showed the yellow line of helium, and 
after six days the whole helium spectrum was observed. 
In order to be certain that the helium, coming possibly 
from some other source, had not diffused through the 
thin walls of the tube A, the emanation was pumped 
out and helium substituted. No trace of helium could 
be observed in the vacuum tube after several days, 
showing that the helium observed in the first experi- 
ment must have originated from the alpha particles 
which had been propelled through the thin glass tube 
into the outer tube.* 

Most of the alpha particles are propelled with such 
force that they penetrate some distance into the walls 
of the outer tube and some of these gradually diffuse 
out into the exhausted space. The presence of helium 
in the spectrum tube can be detected after a shorter 
interval if a thin cylinder of lead is placed over the 
emanation tube, since the particles fired into the lead 
diffuse out more rapidly than from glass. 

A still more definite proof of the identity of the alpha 
particle with the helium atom was obtained by remov- 
ing the outer glass tube T and placing a cylinder of lead 
over the emanation tube in the open air. Helium was 



NATURE OF THE ALPHA PARTICLE 29 

always detected in the lead after it had remained sev- 
eral hours over the thin tube containing a large quantity 
of the emanation. In order to test for the presence of 
helium in the lead, the gases present were released by 
melting the lead in a closed vessel. There can thus be 
no doubt that the alpha particle becomes a helium 
atom when its positive charge is neutralized. 

Thus the chemist was afforded the experience of the 
building up of at least one element under his observa- 
tion, and both the analysis and synthesis of matter 
have been revealed through the discoveries of radio- 
activity. 

It is of interest at this point to learn something of 
the history of helium and its occurrence. In 1868 
Discovery of there was discovered by Janssen and Lock- 
Helium ver a b r ight yellow line in the spectrum of 
the sun's chromosphere. Because of its origin the 
name helium was given to the supposed new element 
causing it. Later it was found in the spectra of many of 
the stars, and because of its predominance in some of 
these they were called helium stars. Its existence on 
our planet was not detected for nearly thirty years. 

In 1895, in connection with the discovery of argon 
in the atmosphere, a search was made to see if the latter 
element could be obtained from mineral sources. In 
analyzing certain uranium minerals Hillebrand had 
found considerable quantities of a gas which he took to 
be a peculiar form of nitrogen. Ramsay made a further 
examination of the gas coming from these minerals and 



30 RADIO-ACTIVITY 

the spectroscope revealed the yellow line of helium, 
thus at last proving the presence of this element on the 
earth. It is known now to be present in thorium min- 
erals, in the waters of radio-active wells, and in minute 
amounts in the atmosphere. Its occurrence in every 
case, in the light of the experiment described above, 
would seem to be due to the presence of radio-active 
changes. 

Helium, on account of its chemical inactivity and 
physical properties, is classed along with argon, neon, 
character krypton, and xenon in the zero group of the 
istics of Periodic System, and forms with them the 
monatomic, inert gases. In this class are 
now placed also the three radio-active gases, emanat- 
ing respectively from radium, thorium, and actinium. 
These are generally known as radium emanation, tho- 
rium emanation, and actinium emanation. The first 
mentioned was once called niton. Emanium was the 
name originally proposed by Giesel for the body now 
known as actinium. 

The calculated rate of production of helium in the 
series in equilibrium with one gram of radium is 158 
cubic millimeters per year. This corresponds quite 
well with the experimental results. 

Some of the more important atomic and radio-active 
constants are given in the following table. They are 
Table of recorded here to show how helpful the study 
Constants o f ra dio-activity has been in working out 
the composition of matter, and to give some idea of 



NATURE OF THE ALPHA PARTICLE 31 

the magnitude of the numbers and the minuteness 
of the quantities dealt with. 

Electric charge carried by each H atom in 

electrolysis 4.65 x 1Q- 10 e.s. 1 

Electric charge carried by each a particle 9.3 x lO" 10 e.s. 

Number of atoms in 1 gram of H 6.2 x 10 23 

Mass of 1 atom of H 1.6 x 10~ 24 gram 

Number of molecules per cc. of any gas at 

standard pressure and temperature 2.72 x 10 19 

Number of a particles expelled per second 

per gram of radium itself 3.6 x 10 10 

Number of a particles expelled per second 

per gram of radium in equilibrium with 

its products 14.3 x 10 10 

1 The expression 10- 10 means multiplying by .000,000,000,1; 10 10 means 
multiplying by 10,000,000,000.^ 



CHAPTER V 
* THE STRUCTURE OF THE ATOM 

A STUDY of the properties of radium will aid in throw- 
ing light upon the question as to the building up of the 
Properties of atom. First to be considered are the usual 
Radium properties which distinguish an elementary 

body. Metallic radium has been prepared by a method 
similar to that used in the preparation of barium. It 
is a pure white metal, melting at 700, and far more 
volatile than barium. It rapidly alters on exposure to 
the air, probably forming a nitride. It energetically 
decomposes water and the product dissolves in the 
water. Its atomic weight is 226. 

Radium forms a series of salts analogous in appear- 
ance and chemical action to those of barium. In the 
course of time they become colored, especially if mixed 
barium salts. The radiations from radium produce 
marked chemical effects in a number of substances. 
Carbon dioxide is changed into carbon, oxygen, and 
carbon monoxide, and the latter is changed into carbon 
and oxygen. Ammonia is dissociated into nitrogen 
and hydrogen; hydrochloric acid into chlorine and 
hydrogen. Oxygen is condensed into ozone. In gen- 
eral, the action upon gases appears to be similar to that' 



THE STRUCTURE OF THE ATOM 33 

of the silent electric discharge. Water is decomposed 
into hydrogen and oxygen. If moist radium chloride 
or a salt of radium containing water of crystallization 
is sealed in a glass tube, the gradual accumulation of 
hydrogen and oxygen will burst the tube. 

The radiations rapidly decompose organic matter 
with the evolution of gases. Thus grease from stop- 
cocks of apparatus used with radium or paraffin will 
give off carbon dioxide. Under an intense alpha radia- 
tion paraffin or vaseline become hard and infusible. 
White phosphorus is changed into red. 

The action upon living tissue is most noteworthy, as 
its possible use as a remedial agent is dependent upon 
this. A small amount of a radium salt enclosed in a 
glass tube will cause a serious burn on flesh exposed to 
it. It therefore has to be handled with care and undue 
exposure to the radiations must be avoided. Cancer 
sacs shrivel up and practically disappear under its 
action. W T hether the destruction of whatever causes 
the cancer is complete is at least open to serious doubt. 

The coagulating effect upon globulin is interesting. 
When two solutions of globulin from ox serum are 
taken and acetic acid added to one while ammonia is 
added to the other, the opalescence in drops of the 
former is rapidly diminished on exposure to radium, 
showing a more complete solution, whereas the latter 
solution rapidly turns to a jelly and becomes opaque, 
indicating a greatly decreased solubility. 

The greater part of the tremendous energy evolved 



34 RADIO-ACTIVITY 

by radium is due to the emission of the alpha particles, 
Energy anc ^ m com P ar i s o n the beta and gamma rays 

Evolved by together supply only a small fraction. This 
energy may be measured as heat. It was 
first observed that a radium compound maintained a 
temperature several degrees higher than that of the 
air around it. The rate of heat production was later 
measured by means of an ice calorimeter and also by 
noting the strength of the current required to raise a 
comparison tube of barium salt to the same tempera- 
ture. Both methods showed that the heat produced was 
at the rate of about 135 gram calories per hour. As 
the emission is continuous, one gram of radium would 
therefore emit about 1,180,000 gram calories in the 
course of a year. At the end of 2000 years it would 
still emit 590,000 gram calories per year. Such a pro- 
duction of energy so far surpasses all experience that 
it becomes almost inconceivable. It is futile to speak 
of it in terms of the heat evolved by the combustion 
of hydrogen, which is the greatest that can be produced 
by chemical means. 

This effect is unaltered at low temperatures, as has 
been tested by immersing a tube containing radium in 
liquid air. It should be stated that these measure- 
ments were made after the radium had reached an 
equilibrium with its products; that is, after waiting at 
least a month after its preparation. The evolution of 
heat from radium and the radio-active substances is, 
in a sense, a secondary effect, as it measures the radiant 



THE STRUCTURE OF THE ATOM 35 

energy transformed into heat energy by the active mat- 
ter itself and whatever surrounds it. Let us repeat, 
therefore, that the total amount of energy pent up in 
a single atom of radium almost passes our powers of 
conception. 

The facts gathered so far justify and necessitate a 

theory which shall satisfactorily explain them, and 

since these phenomena are not caused by 

Necessity for r 

a Disintegra- nor subject to the influence of external 
iory agencies, they must refer to changes taking 
place within the atom in other words, a theory of 
disintegration. In the main, these facts may be 
summed up as the emission of certain radiations from 
known elemental matter: the material alpha particles 
with positive charge, the beta particles or negative 
electrons, and the gamma rays analogous to X rays. 
The emission of these rays results in the production of 
great heat. Then there is the law of transformations ~ 
by which whole series of new elements are generated 
from the original element and maintain a constant equi- ~ 
librium of growth and decay in the series. Lastly, we 
have the production of helium from the alpha particles. *' 

In explanation of these phenomena, Rutherford 
offered the hypothesis that the atoms of certain ele- 
Disintegra- ments were unstable and subject to dis- 
tion Theory integration. The only elements definitely 
known to come under this description are the two 
having atoms of the greatest known mass, thorium 
(232) and uranium (238). 



36 RADIO-ACTIVITY 

The atoms of uranium, for instance, are supposed to 
be not permanent but unstable systems. According 
to the hypothesis, about 1 atom in every 10 18 becomes 
unstable each second and breaks up with a violent 
explosion for so small a mass of matter. One, or pos- 
sibly two alpha particles are expelled with great veloc- 
ity. This alpha particle corresponds to an atom of 
helium with an atomic weight of 4, and its loss reduces 
the original atomic weight to 234 with the formation 
of a new element, having changed properties corre- 
sponding to the new atomic weight. This new element 
is uranium Xi. 

These new atoms are far more unstable than those 
of uranium, and the decomposition proceeds at a new 
rate of 1 in 10 7 per second. So at a definite, measurable 
rate this step wise disintegration proceeds. The explo- 
sions are not in all cases equally violent in going from 
element to element, nor are the results the same. 
Sometimes alpha particles alone are expelled, some- 
times beta, or two of them together, as alpha and beta. 

The new product may remain with the unchanged 
part of the original matter. Thus there would be an 
accumulation of it until its own decay balances its pro- 
duction, resulting eventually in a state of equilibrium. 

In order to explain the electrical and optical prop- 
erties of matter, the hypothesis was made that the 
Constitution atom consisted of positively and negatively 
of the Atom electrified particles. Later it was shown 
that negative electrons exist in all kinds of matter. 



THE STRUCTURE OF THE ATOM 37 

Various attempts were made to work out a model of 
such an atom in which these particles were held in 
equilibrium by electrical forces. The atom of Lord 
Kelvin consisted of a uniform sphere of positive elec- 
trification throughout which a number of negative 
electrons were distributed, and J. J. Thomson has 
determined the properties of this type as to the num- 
ber of particles, their arrangement and stability. 

According to Rutherford, the atom of uranium may 
be looked upon as consisting of a central charge of 
Rutherford's positive electricity surrounded by a number 
Atom Q concentric rings of negative electrons in 

rapid motion. The positively charged centre is made 
up of a complicated system in movement, consisting 
in part of charged helium and hydrogen atoms, and 
practically the whole charge and mass of the atom is 
concentrated at the centre. The central system of the 
atom is from some unknown cause unstable, and one 
of the helium atoms escapes from the central mass as 
an alpha particle. 

There are, confessedly, difficulties connected with 
this conception of the atom which need not, however, 
be discussed here. Much remains to be learned as to 
the mechanics of the atom, and the hypothesis outlined 
above will probably have to be materially altered as 
knowledge grows. Perhaps it may have to be entirely 
abandoned in favor of some more satisfactory solution. 
Until such time it at least suffices as a mental picture 
around which the known facts group themselves. In 



38 RADIO-ACTIVITY 

this picture energy and matter lose their old-time 
distinctness of definition. Discrete subdivisions of 
energy are recognized which may be called charged 
particles without losing their significance. Some of 
these subdivisions charged in a certain way or with 
neutralized charge exhibit the properties of so-called 
matter. 

This conception of the atom would doubtless fail 
of much support were it not for certain experimental 
Scattering of ^ acts which lend great weight to it. Certain 
Alpha Par- suppositions can be based on this theory 
mathematically reasoned out and tested by 
experiment. Predictions thus based on mathematical 
reasoning and afterward confirmed by experiment give 
a very convincing impression that truth lies at the 
bottom. 

The first of these experimental proofs comes under 
the head of what is known as the scattering of the alpha 
particles, a phenomenon which, when first observed, 
proved hard to explain. If an alpha particle in its 
escape from the parent atom should come within the 
influence of the supposed outer electrical field of some 
other atom, it should be deflected from its course and, 
the intensity of the two charges being known, the 
angle of deflection could be calculated. For instance, 
if it came to what might be called a head-on collision 
with the positive central nucleus of another atom, it 
would recoil if it were itself of lesser mass, or would 
propel the other forward if that were the lighter. 



THE STRUCTURE OF THE ATOM 39 

The experiment is carried out by placing a thin metal 
foil over a radio-active body, as radium C, which expels 
alpha particles with a high velocity, and counting the 
number of alpha particles which are scattered through 
an angle greater than 90 and so recoil toward their 
source. This has been done by a number of investi- 
gators and it has been found that the angle of scattering 
and the number of recoil particles 'depend upon the 
atomic weight of the metal used as foil. For example, 
if gold is used, the number of recoil atoms is one in some- 
thing less than 8,000. 

Taking the atomic weight of gold into consideration, 
Rutherford calculated mathematically that this was 
about the number which should be driven backward. 
But he went further and calculated also the number 
which should be returned by aluminum, which has an 
atomic weight of only about one-seventh that of gold. 
Two investigators determined experimentally the num- 
ber for aluminum and their results agreed with Ruther- 
ford's calculations. 

The metals from aluminum to gold have been ex- 
amined in this way. The number of recoil particles 
increases with the atomic weight of the metal. Com- 
paring experiment with theory, the central charge in 
an atom corresponds to about one-half the atomic 
weight multiplied by the charge on an electron, or, as it 
is expressed, J Ae. v 

There is only one lighter atom than helium, namely, 
hydrogen, which has a mass only one-fourth as great. 



40 RADIO-ACTIVITY 

When alpha particles are discharged into hydrogen, a 
few of the latter atoms are found to be propelled to a 
distance four times as great as that reached by the 
alpha particles. 

Parallel with the experiments mentioned, there is 
what is called the stopping power of substances. This 
stopping means the depth or thickness of a substance 
Power of necessary to put a stop to the course of the 
alpha particles. This gives the range of 
the alpha particles in such substances and is connected 
in a simple way with the atomic weight, that is, it is 
again fixed by the mass of the opposing atom. This 
stopping power of an atom for an alpha particle is 
approximately proportional to the square root of its 
atomic weight. 

Considering gases, for instance, if the range in hydro- 
gen be 1, then the range in oxygen, the atomic weight 
of which is 16, is only V^ or J. Generally in the case 
of metals the weight of matter per unit area required 
to stop the alpha particle is found to vary accord- 
ing to the square root of the atomic weight of the 
metal taken. 



CHAPTER VI 

RADIO-ACTIVITY AND CHEMICAL THEORY 

IT can easily be seen that the revelations of radio- 
activity must have a far-reaching effect upon chemical 
theory, throwing light upon, and so bringing 
upon chem- nearer, the solution of some of the problems 
tory which have been long discussed without 
arriving at any satisfactory solution. The so-called 
electro-chemical nature of the elements will certainly 
be made much clearer. The changes in valence should 
become intelligible and valence itself should be ex- 
plained. A fuller understanding of the ionization of 
electrolytes also becomes possible. As these matters 
are debatable and the details are still unsettled, it is 
scarcely appropriate to give here the hypotheses in 
detail or to enter into any discussion of them. But 
the promise of solution in accord with the facts is 
encouraging. 

Such progress has been made, however, in regard to 
a better understanding of the Periodic System that the 
The Periodic n ^w facts and their interpretation may well 
System j^ gi ven N O reliable clue to the meaning 

of this system and the true relationship between the 



42 RADIO-ACTIVITY 

elements had been found up to the time when new 
light was thrown upon it by the discoveries of radio- 
activity. The underlying principle was unknown and 
even the statement of what was sometimes erroneously 
called the Periodic Law was manifestly incorrect and 
its terms were ignored. 

The ordinary statement of the fundamental prin- 
ciple of the Periodic System has been that the 
Basis of the P r P er ti es of the elements were periodic 
Periodic Sys- functions of the atomic weights, and that 
when the elements were arranged in the 
order of their atomic weights they fell into a natural 
series, taking their places in the proper related 
groups. 

In accepting this, the interpretation of function was 
both unmathematical and vague, and the order of the 
atomic weights was not strictly adhered to but unhesi- 
tatingly abandoned to force the group relationship. 
Wherever consideration of the atomic weight would 
have placed an element out of the grouping with 
other elements to which it was clearly related in 
physical and chemical properties, the guidance of these 
properties was accepted and that of the atomic 
weights disregarded. Such shif tings are noted in the 
cases of tellurium and iodine; cobalt and nickel; 
argon and potassium. It was most helpful that, fol- 
lowing the order of atomic weights, the majority of 
the elements fell naturally into their places. Otherwise 
the generalization known as the Periodic System might 



RADIO-ACTIVITY AND CHEMICAL THEORY 43 

have remained for a long time undiscovered and 
the progress of chemistry would have been greatly 
retarded. 

It is evident that the order of the elements is deter- 

mined by something else than their atomic weights. 

From the known facts of radio-activity it 

Influence of .,... 

Positive would seem that this determining factor is 

the positive nucleus. And this nucleus also 
determines the mass or weight of the atom. Taking 
the elements in their order in the Periodic Series and 
numbering the positions held by them in this series as 
1, 2, 3, etc., we get the position number or what is 
called the atomic number. This designates the order or 
position of the element in the series. We must learn 
that this number marks a position rather than a 
single element, a statement which will be explained 
later. 

Since the atomic weight is unreliable as a means of 

settling the position of an element in the series and so 

fixing its atomic number, how is this number 

Determina- , . ,* >-. 

turn of the to be determined : (Jt course, one answer 



Nmnber to ^ s question is that we may rely upon a 
consideration of the general properties, as 
has been done in the past. Fortunately, other methods 
have been found by which this may be confirmed. 
For instance, the stopping and scattering power of the 
element for alpha particles has been suggested and suc- 
cessfully used. 

A most interesting method is due to Moseley's 



44 RADIO-ACTIVITY 

observations upon the X-ray spectra of the various 
Use of x- elements. It has been found that crystals, 
Ray Spectra sucn as those of quartz, have the power of 
reflecting and defining the X rays. The spectra given 
by these rays can be photographed and the wave lengths 
measured. These X rays are emitted by various sub- 
stances under bombardment by the cathode rays 
(negative electrons) and have great intensity and very 
minute wave lengths. Moseley made use of various 
metals as anti-cathodes for the production of these 
rays. These metals ranged from calcium to zinc in 
the Periodic System. In each case he observed that 
two characteristic types of X rays of definite intensity 
and different wave lengths were emitted. From the 
frequency of these waves there is deduced a simple 
relation connected with a fundamental quantity which 
increases in units from one element to the next. This 
is due to the charge of the positive central nucleus. 
The number found in this way is one less than the 
atomic number. Thus the number for calcium is 19 
instead of 20 and that for zinc is 29 instead of 30. So, 
by adding 1 to the number found the atomic number is 
obtained. 

The atomic weight can usually be followed in fixing 
the atomic number, but where doubt exists the method 
just given can be resorted to. Thus doubt arises in 
the case of iron and nickel and cobalt. This would be 
the order according to the atomic weights. The 
X-ray method gives the order as iron, cobalt, and 



RADIO-ACTIVITY AND CHEMICAL THEORY 45 

nickel, and this is the accepted order in the Periodic 
System. 

On studying the properties of the elements in a trans- 
formation series in connection with the ray emission 
which produced them, it was seen that these 

Changes \ ' . 

Caused by properties were determined in each case by 

Ray Emission ^ nature Qf ^ rfty emiUed from the 

preceding transformation product or parent element. 

Each alpha particle emitted means a loss of 4 in the 

atomic weight. This is the mass of a helium atom. 

Thus from uranium with an atomic weight 

Atomic . 

Weight of 238 to racjium there is a loss ot three 

alpha particles. Therefore, 12 must be 
subtracted from 238, leaving 226, which agrees closely 
with the atomic weight of radium as actually determined 
by the ordinary methods. Uranium Xi, then, would 
have an atomic weight of 234 and that of ionium would 
be 230. The other intermediate elements, whose forma- 
tion is due to the loss of beta particles only, show no 
decrease in atomic weight. 

From uranium to lead there is a loss of 8 alpha par- 
ticles, or 32 units in atomic weight. This would give 
Lead the for the final product an atomic weight of 
End Product 206 The atom i c we ight of lead is 207.17. 
It is not at all certain that the final product of this 
series is ordinary lead. The facts are such that they 
would lead one to think that it is not. It is known 
only that the end product would probably be some ele- 
ment closely resembling lead chemically and hence 



46 RADIO-ACTIVITY 

difficult or impossible to separate from it. Several 
accurate determinations of lead coming from uranium 
minerals, which always carry this element and in an 
approximately definite ratio to the amount of uranium 
present, show atomic weights of 206.40; 206.36; and 
206.54. Even the most rigid methods of purification 
fail to change these results. The lead in these minerals 
might therefore be considered as coming in the main 
from the disintegration of the uranium atom and, 
though chemically resembling lead, as being in reality 
a different element with different atomic weight. 

Furthermore, in the thorium series 6 alpha particles 
are lost before reaching the end product, which again 
is perhaps the chemical analogue of lead. The atomic 
weight here should be 232 less 24, or 208. Determina- 
tions of the atomic weight of lead from thorite, a tho- 
rium mineral nearly free from uranium, gave 208.4. 

The end product of the actinium series is also an 
element resembling lead, but both the beginning and 
ending of this series are still in obscurity. 

The loss of 4 units in the atomic weight of an element 

on the expulsion of an alpha particle is 

Position in accompanied by a change of chemical 

the Periodic properties which removes the new element 

System . i 

two groups toward the positive side in the 
Periodic System. 

Thus ionium is so closely related to thorium and so 
resembles it chemically that it is properly classed along 
with thorium as a quadrivalent element in the fourth 



RADIO-ACTIVITY AND CHEMICAL THEORY 47 

group. Ionium expels an alpha particle and becomes 
radium, which is a bivalent element resembling barium 
belonging to the second group. Radium then expels 
an alpha particle and becomes the gas, radium emana- 
tion, which is an analogue of argon and belongs to the 
zero group. Other instances might be cited which go 
to show that in all cases the loss of an alpha particle 
makes a change of two places toward the left or positive 
side of the System. 

The loss of a beta particle causes no change in the 

atomic weight but does cause a shift for each beta par- 

ticle of one group toward the right or nega- 

from LOSS of tive side of the System. Two such losses, 



" then, will counterbalance the loss of an 
alpha particle and bring the new element 
back to the group originally occupied by its progenitor. 
Thus uranium in the sixth group loses an alpha particle 
and the product UXi falls in the fourth group. One 
beta particle is then lost and UX 2 belonging to the 
fifth group is formed. With the loss of one more beta 
particle the new element returns to the sixth group from 
which the transformation began. 

The table on page 48, as adapted from Soddy, affords 
a general view of these changes. 

An examination of the table will show a number of 

different elements falling in the same position in a 

isoto es group of the Periodic System irrespective 

of their atomic weights. These are chem- 

ically inseparable so far as the present limitations of 



48 RADIO-ACTIVITY 



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RADIO-ACTIVITY AND CHEMICAL THEORY 49 

chemical analysis are concerned. Even the spectra 
of these elements seem to be identical so far as 
known. This identity extends to most of the physical 
properties, but this demands much further investi- 
gation. For this new phenomenon Soddy has sug- 
gested the word isotope for the element and isotopic 
for the property, and these names have come into 
general use. 

Manifestly, we have come across a phenomenon here 
which quite eliminates the atomic weight as a deter- 
mining factor as to position in the Periodic or Natural 
System or of the elemental properties in general. All 
of the properties of the bodies which we call elements, 
and consequently of their compounds and hence of 
matter in general, seem to depend upon the balance 
maintained between the charges of negative and positive 
electricity which, according to Rutherford's theory, go 
to make up the atom. 

It is evident that any study of chemical phenomena 
and chemical theory is quite incomplete without a 
study of radio-activity and the transformations which 
it produces. 

In concluding this outline of the main facts of radio- 
activity, it is of interest to discuss briefly the presence 
Radio-activ- of radio-active material on this planet and 
ity in Nature in the stars> Facts enougn have been 

gathered to show the probable universality of this 
phenomenon of radio-activity. Whether this means 
solely the disintegration of the uranium and thorium 



50 RADIO-ACTIVITY 

atoms, or whether other elements are also transformed 
under the intensity of the agencies at work in the uni- 
verse, is of course a question as yet unsolved. 

The presence of uranium and thorium widely dis- 

tributed throughout the crust of the earth would lead 

to the conclusion that their disintegration 

Products in products would be found there also. Vari- 



ous rocks of igneous origin have been exam- 
ined revealing from 4.78 X 10~ 12 to 0.31 
X 10 ~ 12 grams of radium per gram of the rock. Aque- 
ous rocks have shown a lesser amount, ranging from 
2.92 X 10- 12 to 0.86 X 10- 12 grams. As the soil is 
formed by the decomposition of these rocks, radium is 
present in varying amounts in all kinds of soil. 

As radium is transformed into the gaseous emanation, 
this will escape wherever the soil is not enclosed. For 
~ n . instance, a larger amount of radio-activity 
Air and Soil is found in the soil of caves and cellars than 
in open soils. If an iron pipe is sunk into 
a soil and the air of the soil sucked up into a large elec- 
troscope, the latter instrument will show the effect of 
the rays emitted and will measure the degree of activity. 
Also the interior of the pipe will receive a deposit of 
the radio-active material and will show appreciable 
radio-activity after being removed from the soil. 

This radium emanation is dissolved in the soil waters, 
wells, springs, and rivers, rendering them more or less 
radio-active, and sometimes the muddy deposit at the 
bottom of a spring shows decided radio-activity. 



RADIO-ACTIVITY AND CHEMICAL THEORY 51 

The emanation also escapes into the air so that many 
observations made in various places show that the 
radium emanation is everywhere present in the atmos- 
phere. Neither summer nor winter seems to affect 
this emanation, and it extends certainly to a height of 
two or three miles. Rain, falling through the air, dis- 
solves some of the emanation, so that it may be found 
in freshly-fallen rain water and also in freshly-fallen 
snow. Radio-active deposits are found upon electri- 
cally charged wires exposed near the earth's surface. 

As helium is the resulting product of the alpha par- 
ticles emitted by the emanation and other radio-active 
bodies, it is found in the soil air, soil waters, and 
atmosphere. 

Average measurements of the radio-activity of the 
atmosphere have led to the calculation that about one 
gram of radium per square kilometer of the earth's sur- 
face is requisite to keep up the supply of the emanation.. 

A number of estimates have been given as to the 
heat produced by the radio-active transformations going 
on in the material of this planet. Actual data are 
scarce and mere assumptions unsatisfactory, so little 
that is worth while can be deduced. It is possible that 
this source of heat may have an appreciable effect upon 
or serve to balance the earth's rate of cooling. 

Meteorites of iron coming from other celestial bodies 
Cosmicai have not shown the presence of radium. 
Radio-activity Aerolites or stone meteorites have been 
found to contain as much as similar terrestrial rock. 



52 RADIO-ACTIVITY 

Since the sun contains helium and some stars 
show its presence as predominating, this suggests the 
presence of radio-active matter in these bodies. In 
addition, the spectral lines of uranium, radium, and 
the radium emanation have been reported as being 
found in the sun's spectrum and also in the new star, 
Nova Geminorum 2. These observations await further 
investigation and confirmation. So far as the sun's 
chromosphere is concerned, the possible amount of 
radium present would seem to be very small. If this 
is true, radio-active processes could have little to do 
with the sun's heat. The statement is made by 
Rutherford that indirect evidence obtained from the 
study of the aurora suggests that the sun emits rays 
similar in type to the alpha and beta rays. Such rays 
would be absorbed, and the gamma rays likewise, in 
passing through the earth's atmosphere and so escape 
ordinary observation. All of this is but further evi- 
dence of the unity of matter and of forces in the 
universe. 



INDEX 



Actinium, discovery of, 6 
Activity, induced, 17 
Alpha particles, effect of loss on 
Atomic Weight, 45 

electrical charge of, 26 

form helium, 27 

nature of, 25 

penetrating power of, 39 

position of element changed by 
its loss, 46 

recoil, 39 

scattering of, 38 

solid, 26 
Atom, constitution of, 36 

Kelvin's, 37 

models of, 37 

Rutherford's, 37 
Atomic number, determination of, 43 

Becquerel's experiments, 2 
Beta particles, change in position of 
element by loss of, 47 

Chalcolite, natural and artificial, 4 
Constants, table of, 31 
Curie unit, 22 

Disintegration of the element, 25 
Disintegration series, 24 
Disintegration theory, 35 

Electroscope, 12 
Equilibrium series, 22 



Helium, characteristics of, 30 
discovery of, 29 

Ionium, discovery of, 6 
lonization, application of electric 
field to, 10 

experimental confirmation, 9 
lonization of gases, 7 

theory of, 8 

Ions, size and nature of, 10 
Isotopes, 47 

Lead, atomic weight varies with 

source, 45 
radio-active, 6 
the end product, 45 
Life-periods of radio-active bodies, 
21 

Periodic system, 41 

basis of, 42 

Polonium, discovery of, 4 
Positive nucleus, influence of, 43 
Potassium, radio-activity of, 3 

Radiations, action on phosphores- 
cent bodies, 13 

action on photographic plates, 11 
discharge electrified bodies, 12 
magnetic deflection of, 14 
measurements of, 15 
penetrating power of, 13, 15 



54 



. 

Radio-active bodies, elemental na- 
ture of, 20 

examination of, 20 

life periods of, 21 
Radio-activity, an atomic property, 3 

cosmical, 51 

influence on chemical theory, 41 

products in atmosphere, 51 

products in earth's crust, 50 

products in soil waters, 50 
Radium, action on organic matter, 
etc., 33 

amount in pitchblende, 5 

discovery of, 5 

emanation, 22 

energy evolved by, 34 

properties of, 5, 32 
Rays, alpha, 15, 16, 26 

beta, 15, 16 



INDEX 



gamma, 15, 16 
identification of, 16, 25 
magnetic deflection of, 14 
photographing track of, 10 
types of, 14 
Rubidium, radio-activity of, 3 

Spinthariscope, 13 

Stopping power of substances, 39 

Thorium X, discovery of, 18, 21 

Uranium atom, disintegration of, 36 
Uranium minerals, radio-activity of ,3 
Uranium X, discovery of, 17, 21, 23 

X-ray spectra, 44 
Zinc sulphide screen, 13 













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UNIVERSITY OF CALIFORNIA LIBRARY